In general, during a gear shift, such feedback control to a predetermined differential rotation is conducted to slip a lock-up clutch (so-called slip lock-up control) and thereby suppress a shift shock.
On the other hand, upon a non-gear shift, except for the case where a large driving force is required, engagement of the lock-up clutch is carried out to thereby eliminate slippage in a torque converter and improve fuel economy, as generally performed.
Further, in recent years, with the progress of a multi-stage configuration of an automatic transmission, a gear shift is frequently carried out. Therefore, in order to instantaneously execute the slip lock-up control, in a case where slippage of the lock-up clutch is not necessary, for instance, upon the non-gear shift, control of a lock-up engagement capacity is also carried out such that the lock-up clutch is brought into a state immediately before slippage occurs (hereinafter referred to as a zero slip state) (for instance, see Patent Literature 1).
However, for the reasons described below, it is significantly difficult to control the lock-up engagement capacity such that the lock-up clutch is brought into the state immediately before slippage occurs (the zero slip state). There is a problem that when during the non-gear shift, the lock-up clutch is brought into a state in which the lock-up clutch has a capacity larger than necessary (a substantially engagement state), a start of the slip lock-up control during a gear shift is delayed so that a shift shock is caused.
(a) The zero slip state is a state in which engine rotation speed Ne and turbine rotation speed Nt are equal to each other. By detecting the rotation speed, it is recognized that there occurs no clutch slippage. However, the mere detection of the rotation speed is insufficient to determine whether the lock-up engagement capacity is excessively large or the lock-up engagement capacity is appropriate.
(b) Although as generally performed, when transition of the lock-up clutch from the release state to the engagement state is carried out, feedback control is performed while gradually reducing a target slip amount of the lock-up clutch, it is difficult to control actuation (initiation) of a mechanism of the lock-up clutch because a return spring is not used in the mechanism of the lock-up clutch. For instance, when the lock-up clutch is actuated, an inertia force is exerted on the lock-up clutch so that the lock-up clutch is occasionally allowed to mechanically move and come into the engagement state in a stage in which the target slip amount is still large.
(c) It is possible to estimate a timing at which the lock-up engagement capacity is started to generate to a certain extent by using engine torque Te, engine rotation speed Ne and the like (Te-τNe2). However, engine torque Te and engine rotation speed Ne suffer large influence of disturbance, and there is a delay in detection of engine torque Te and engine speed Ne. Therefore, it is significantly difficult to hold the lock-up clutch at the capacity immediately before the lock-up clutch starts to slip with high accuracy.